Deformation of Ultra-Thin Diamond-Like Carbon Coatings

ORIGINAL PAPER

Deformation of Ultra-Thin Diamond-Like Carbon CoatingsUnder Combined Loading on a Magnetic Recording Head

Michael R. Price • Andrey Ovcharenko •

Raj Thangaraj • Bart Raeymaekers

Received: 12 September 2014 / Accepted: 10 October 2014 / Published online: 10 January 2015

� Springer Science+Business Media New York 2014

Abstract Ultra-thin diamond-like carbon (DLC) coatings

are used in precision engineering applications, such as

magnetic storage devices, to protect intricate structures

from wear and corrosion. A DLC coating typically consists

of hard amorphous carbon in combination with an inter-

layer such as silicon (Si), to improve adhesion to the sub-

strate material. Deformation and delamination of these

coatings, even in part, could expose the substrate material

and compromise its integrity and functionality. We have

implemented a molecular dynamics model to quantify the

strength of the interface between an ultra-thin tetrahedral

amorphous carbon coating, a Si layer, and a permalloy

(NiFe) substrate, under combined normal and tangential

loading that mimics accidental contact between the

recording head and the disk of a hard drive. We have

evaluated the effect of the thickness of the different coating

layers on deformation and interfacial strength of the coat-

ing during combined loading. The results indicate that

deformation occurs primarily in the Si layer, and at the

interface between the Ni–Si and the Si–C layers. Perma-

nent separation of the Si and ta-C layers is observed, which

gradually increases with multiple combined loading cycles.

We find that increasing the Si and carbon layer thickness

strengthens the DLC coating. However, increasing the

carbon layer thickness has a larger effect on coating

strength than increasing the Si layer thickness.

Keywords Diamond-like carbon � Recording head �Magnetic storage � Ultra-thin films � Interfacial strength

1 Introduction

One of the primary drivers of the magnetic storage industry

is the desire for increased storage density [1]. To accom-

modate this, the magnetic spacing between the read/write

element of the recording head and the magnetic layer of the

disk must be decreased [2]. Figure 1 shows a schematic of

the head–disk interface (HDI). The intricate read/write

structures of the recording head and the delicate magnetic

layer of the disk are protected from accidental contact

during hard drive operation by a tetrahedral amorphous

carbon (ta-C) and an amorphous carbon (a-C) coating,

respectively. The terms ta-C, a-C, and diamond-like carbon

(DLC) are sometimes used interchangeably in the literature

[3]. Following Casiraghi et al. [3] and Robertson [4], we

define ta-C and a-C to be an amorphous form of carbon

with 40–90 % and \40 % sp3 content, respectively, with

both ta-C and a-C considered to be a type of DLC. In recent

years, the decrease in the flying height between the

recording head and the magnetic disk has turned attention

to these protective DLC coatings [5]. Removal of the ta-C

coating of the recording head, even in part, may expose the

read/write element to corrosion and compromise its func-

tion and reliability. A silicon (Si) or silicon nitride (SiN)

layer may be used to improve adhesion between the hard

ta-C layer and the recording head substrate material [6].

Experimental data show that wear and delamination of the

DLC coating occur first at the pole tip area of the recording

head, and specifically at the permalloy (NiFe) write shield

[7, 8]. The pole tip area consists of the read/write elements

and shields and is the portion of the head closest to the disk

M. R. Price � B. Raeymaekers (&)

Department of Mechanical Engineering, University of Utah,

Salt Lake City, UT 84112, USA

e-mail: [email protected]

A. Ovcharenko � R. Thangaraj

Western Digital Corporation, San Jose, CA 95138, USA

123

Tribol Lett (2015) 57:3

DOI 10.1007/s11249-014-0449-2

during reading and writing. As the thickness of the Si and

ta-C layers is reduced to further decrease the magnetic

spacing, wear and delamination of these coatings become

of increased concern. Thus, an understanding of the

deformation and interfacial strength of the protective

coating layers of the recording head as a function of

operating conditions and coating design parameters is

needed. We use a molecular dynamics (MD) approach to

simulate the interaction between the different coating lay-

ers of a small portion of the HDI indicated by the rectangle

in Fig. 1.

Several published studies document and review the

mechanical properties of DLC coatings [4, 9–14], with

some publications primarily focusing on DLC coatings for

hard disk drive (HDD) applications [12–14]. Other works

have studied specific aspects of the mechanical properties

of the DLC coatings in HDDs. For instance, Prabhakaran

and Talke [15] quantified wear of DLC coatings on mag-

netic recording heads by measuring the change in depth of

ion-milled trenches on the surface of the head. They found

good correlation of their results with scratch test results.

Lee et al. [16] used a capacitive precision actuator to

perform nanoindentation of sub-10-nm-thick DLC films

deposited on a glass substrate. They measured the hardness

of the DLC coating as a function of indentation depth and

found an average hardness of 10–15 GPa when the

indentation depth remained far enough from the glass

substrate. Zhong et al. [17] investigated the mechanical

properties and oxidation and corrosion resistance of ultra-

thin a-C coatings. They determined a critical minimum

coating thickness of 1.4 nm to ensure wear resistance under

normal HDD operation, because a coating thinner than

1.4 nm did not significantly reduce the depth of the wear

scars compared to those on an uncoated surface. Yasui

et al. [18] also characterized sub-2-nm-thick protective

DLC coatings on the recording head. They found that the

critical normal load for wear resistance during a scratch test

depends on the substrate material and the thickness of the

DLC coating but not on the deposition technique of the

coating.

As the thickness of the ultra-thin protective DLC coat-

ings is reduced further to accommodate decreasing the

magnetic spacing between the recording head and the

magnetic disk, experimental characterization becomes

increasingly difficult. Consequently, several authors have

resorted to stochastic simulation tools, such as MD, to

study the mechanical behavior and properties of ultra-thin

DLC coatings [19–21]. Evaluating the tribological prop-

erties of a-C, ta-C, and thin diamond films has been the

objective of several other MD studies. Ma et al. [22]

investigated sliding of a-C coatings against a diamond

counterface. They found that the low friction coefficient of

the interface between a-C and diamond resulted from

shear-induced graphitization of the a-C surface, migration

of graphitized carbon layers across the sliding interface,

and relative motion between the graphitized layers. Gao

et al. [23] evaluated the tribological properties of a-C

surfaces sliding against a diamond counterface as a func-

tion of the properties of the a-C layer. They found that the

tribological behavior is highly dependent on the sp3/sp2

ratio of the a-C film. Wang and Komvopoulos [24] con-

firmed this observation. Additionally, Glosli et al. [25]

emphasized, based on the MD results, that for ultra-thin

coatings \5 nm thick, the mechanical properties are

dominated by interfacial phenomena. The primary focus of

these MD studies [22–25] is the interaction between the

amorphous carbon layer and a rigid diamond counterface.

Although minimizing wear and delamination is of crit-

ical importance when decreasing the thickness of DLC

coatings on magnetic recording heads, no publications

seem to exist that use MD to model and quantify the

deformation and interfacial strength of the different layers

of the DLC coating of a recording head. Several authors

have studied deformation of ultra-thin coatings and

delamination from the substrate [26, 27]. However, these

studies do not model the materials, interfaces, or operating

conditions that occur in a HDD. Thus, the objective of this

paper is to evaluate the interfacial strength between the ta-

C coating, Si layer, and permalloy substrate as a function of

thickness of the different coating layers and the contact

pressure during combined normal and tangential loading of

the recording head against the magnetic disk. We imple-

ment an MD model of a small three-dimensional portion of

the HDI, simulating sliding contact between the DLC

coating of the recording head and the disk, and we study

the interfacial strength of the different coating layers under

combined normal and tangential loading. While tailored to

the HDI, this study also attempts to provide a general

approach and framework for quantifying the strength of the

interface between ultra-thin DLC coatings and a substrate.

Disk substrateMagnetic layer

Carbon overcoatLubricant

Magnetic spacingCarbon overcoat

Write coilsGMR element

Head substrate

ShieldsTop pole

Flying height

Si layer

Fig. 1 Schematic of the head–disk interface, showing the different

material layers and structures of the recording head and magnetic

disk. The molecular dynamics model, shown as a magnified view,

simulates the portion of the recording head indicated by the box

3 Page 2 of 9 Tribol Lett (2015) 57:3

123

2 Methods

2.1 Model

Figure 2 shows the MD model, which consists of a small

three-dimensional section of a magnetic recording head, as

indicated in Fig. 1, sliding against a hydrogen-terminated

diamond counterface. The rigid hydrogen-terminated dia-

mond counterface is used in place of a magnetic disk to

reduce the computational cost of the MD simulations and

because we focus on deformation of the recording head.

However, the shear stress between the head and disk sur-

faces is similar to that between the recording head and the

hydrogenated diamond. The lubricant layer on the disk is

not included in our model, and, thus, we simulate a worst-

case scenario of contact with lubricant depletion. The

recording head substrate consists of a 47 A Ni layer. While

Ni has different magnetic properties than NiFe, its

mechanical properties including hardness, Young’s modu-

lus, and Poisson ratio are similar [28–31]. Hence, the

amount of deformation induced in the substrate, and con-

sequently in the ta-C and Si layers, is not significantly

affected by using Ni instead of NiFe. The Ni substrate is

covered with a Si layer of thickness 3 B tSi B 9 A and a

ta-C layer of thickness 9 B tC B 18 A. The thickness of

the Si and ta-C layers is varied in this study by removing

atoms from the middle of the respective layers, thereby

ensuring that the ta-C surface, and the interface between

ta-C and Si, and Ni and Si are consistent for coatings of

different thickness. The sp3 content of the ta-C coating is

65 % and is constant throughout this study. The ta-C

coating is formed using a heating and quenching procedure

[23, 24]. This process ensures a uniform sp3 content

throughout the ta-C coating. The a-Si layer is created using

a similar technique. The simulation box measures

42.24 9 100.00 9 21.12 A in x-, y-, and z-directions,

respectively, and the model contains between 8,075 and

9,896 atoms, depending on the thickness of the different

coating layers. The boundary conditions of the MD model

are periodic in the x- and z-directions. The three outermost

Ni atom layers in the y-direction are held rigid, and the

three adjacent atom layers are maintained at 300 K using a

Langevin thermostat to mimic the presence of surrounding

bulk material. The hydrogen-terminated diamond counter-

face is held rigid throughout the simulation. The remaining

atoms are free to move according to the micro-canonical

ensemble. The interatomic interactions are implemented

with the following potentials: MEAM [27, 32] for Ni–Ni

and Ni–Si interactions; Tersoff [33] for Si–Si, Si–C, and

C–C interactions; and AIREBO [34] for C–H and H–H

interactions. The potentials at the interface of coating

layers, such as between Tersoff and AIREBO for the C–C

and C–H interactions, overlap. The pair coefficients are

modified such that when two potentials contribute to the

pairwise energy between a single pair type, such as C–C

interactions, only one interatomic potential contributes a

nonzero energy value. This allows the energy to be cal-

culated correctly even when second-nearest neighbors are

modeled using a different potential than first-nearest

neighbors. Due to complete separation of the Ni and ta-C

layers by the Si layer, there are no first-nearest neighbor

Ni–C interactions even in the case of the thinnest silicon

layer. However, second-nearest neighbor interactions have

been included in the model. We have used the Sandia

LAMMPS code to perform the MD simulations [35]. A

time step of 0.25 fs is used, and equilibration at 300 K for

10 ps is performed for all simulations.

2.2 Simulation Procedure

Figure 3 shows combined normal and tangential loading of

the recording head on the disk, which simulates accidental

contact. The diamond counterface moves relative to the

recording head, at a constant speed of 75 m/s in the x-

direction (step 1), which is similar to the highest relative

velocity observed between recording head and disk in high-

end server HDDs. The recording head is then loaded

against the moving counterface until the desired contact

pressure is reached (step 2), calculated as the ratio of the

normal load between the recording head and the rigid

counterface and the cross-sectional area of the simulation

box (x–z plane). The moving counterface continuously

slides against the head, resulting in combined normal and

tangential loading (step 3). After sliding contact and

Fig. 2 Molecular dynamics model of a small portion of the head–

disk interface, showing the different material layers of the recording

head and the hydrogen-terminated diamond counterface, and their

respective thickness. The magnified inset shows the different carbon

hybridizations in the ta-C layer

Tribol Lett (2015) 57:3 Page 3 of 9 3

123

combined loading, the head is separated from the moving

counterface (step 4). The x-velocity of the disk is main-

tained constant until complete separation of head and

counterface is obtained (step 5). We have performed sim-

ulations at a contact pressure pc between the head and the

counterface of 48 and 64 GPa, respectively. pc is realistic

for typical head/disk impact and is calculated based on

measured wear areas and contact loads [8, 36]. They were

also chosen to be slightly below the mean hardness of the

coatings, which was measured to be 67.5 GPa in nanoin-

dentation MD simulations, to give measurable deformation

of the recording head in the short (130 ps) simulation time.

2.3 Deformation Analysis

We quantify deformation of the different coating layers and

their interfaces by evaluating the number and length of the

interatomic bonds of each bond type, throughout the sim-

ulation, relative to their respective equilibrium bond length.

A bond exists between two atoms when their separation is

less than a cutoff distance that falls between their first- and

second-nearest neighbor distances [23]. The cutoff distance

used for determining first-nearest neighbors and, thus,

bonded versus non-bonded interactions is defined as the

distance where the minimum between the first and second

peaks of the radial distribution function (RDF) occurs for

that bond type. Table 1 lists these cutoff values. The bond

length is a function of the load and is quantified as strain.

However, the bond cutoff is not affected by load and

remains unchanged throughout the simulation. The equi-

librium bond length is determined as the location of the

first peak in the RDF. Figure 4 shows the RDF, g(r), of the

C–C bond type as an example. The location of the first

peaks in the RDFs is within 1 % of the equilibrium refer-

ence structures for all atomic interactions except for Si–Si

and C–C interactions. The location of the Si–Si peak

results in an equilibrium bond length 8.7 % larger than

predicted by the reference structure, because the amor-

phous Si layer conforms to the Ni face-centered cubic

(FCC) lattice. The location of the C–C peak at 1.50 A

corresponds to the length of a bond between sp3- and sp2-

hybridized carbon atoms [37], which is the prevailing

structure in the amorphous mixture of sp3- and sp2-

hybridized carbon atoms in the ta-C layer.

We define deformation on the atomic scale as a change

in the number of bonds, with a permanent change in the

number of bonds indicating plastic deformation and a

permanent decrease in the number of bonds between two

coating layers indicating delamination or separation in or

between coating layers. Strain is calculated as the ratio of

the change of the bond length between two atoms Dl and

the equilibrium bond length between those atoms l0. Local

strain in the MD model is determined by overlaying a grid

in the x–y plane on the recording head and calculating the

average strain of all the bonds with x- and y-coordinates

that fall into each grid element, i.e., R(Dl/l0)/Nbonds for each

grid element, where the summation is over the total number

of bonds Nbonds in the grid element. Residual strain is

quantified by calculating the strain in the coating before

loading has occurred. It is a measure of the strain caused by

0 1 2 3 4 5 60

1

2

3

4

5

Distance, r [Å]

Rad

ial d

istri

butio

n fu

nctio

n, g(r)

Bond cutoff

Equilibrium bond length

Fig. 4 Radial distribution function for C–C interactions, for a

recording head coating with tSi = 3 A and tC = 12 A. The values

have been normalized with the maximum value at r = 6 A

Table 1 Bond length and cohesive energy of each of the bond types

in the MD model

Bond

type

Equilibrium

structure

Equilibrium

structure

bond length

(A)

RDF

bond

length

(A)

RDF

bond

cutoff

(A)

Cohesive

energy

[eV]

Ni–Ni FCC 2.49 [27] 2.49 3.10 4.44 [38]

Ni–Si l12 2.41 [27] 2.40 2.77 5.51 [39]

Si–Si Diamond 2.35 [33] 2.56 2.91 4.63 [40]

Si–C 3C–SiC 1.89 [33] 1.87 2.35 6.47 [41]

C–C Diamond 1.54 [4] 1.50 1.96 7.84 [42]

The equilibrium bond length is also given for each bond type

The bond length and bond cutoff distance is determined from the

radial distribution functions (RDF)

1

vx

pc

pc

3

vx

5

vx

2

vx

vy

4

vx

vy

Fig. 3 Schematic of combined normal and tangential loading

procedure between the recording head and disk


123

the difference in bond length and local structure mismatch

near the Ni–Si and Si–C interfaces. Changes in the total

bond energy throughout the head are also used to quantify

deformation. Bond energy is calculated as the summation

of the product of the number of bonds of a particular type

NT multiplied with the cohesive energy of that type ET (see

Table 1), i.e., R(NT*ET).

3 Results and Discussion

3.1 Residual Strain

Figure 5 shows the mean residual strain in the recording

head, prior to loading, as a function of tSi, for different

values of tC. The mean residual strain decreases with

increasing tSi, but is almost independent of tC. The mean

bond length of Si–Si bonds, which is 8.7 % larger than the

theoretical value (see Table 1), and the local strain visu-

alizations (see Fig. 5) indicate that most of the strain is

localized at the Ni–Si and Si–C interfaces. The Si layer

conforms to the amorphous structure of the ta-C layer on

one side and to the FCC lattice of Ni on the other side. As

the thickness of the Si layer decreases, the transition from

Ni to ta-C occurs over a smaller distance, increasing the

local strain in the Si layer and surrounding interfaces due to

the mismatch between the Ni, Si and ta-C structures.

3.2 Deformation During Combined Loading

Figure 6 shows the instantaneous number of bonds for each

bond type in the MD model, normalized with the initial

number of bonds of that type, as a function of time during

the combined loading simulation, for a DLC coating with

tSi = 3 A and tC = 12 A, and for contact pressure

pc = 48 GPa (Fig. 6a) and 64 GPa (Fig. 6b). The five

steps shown in Fig. 3 are represented by three regions in

Fig. 6. Region I corresponds to the initial equilibration and

normal loading steps, region II corresponds to the com-

bined normal and tangential loading, and region III corre-

sponds to the unloading and separation steps. We observe

that deformation occurs primarily in the Ni–Ni, Ni–Si, and

Si–Si bonds during combined normal and tangential load-

ing of the recording head and the rigid, moving counter-

face, because these bond types display the lowest cohesive

energies of the different bond types in the recording head

(see Table 1) and, thus, are easiest to deform. Although

Ni–Ni bonds have the lowest cohesive energy, crystalline

structures show higher intrinsic resistance to deformation

compared to amorphous structures [43]. Hence, we observe

deformation of the Ni–Si and Si–Si bonds before Ni–Ni

bonds. As the load on the Ni substrate increases, the dis-

tance between second-nearest neighbor atoms is reduced

Fig. 5 Mean residual strain in the recording head as a function of tSi

for different values of tC. The corresponding plots of local residual

strain are shown for the coatings with tC = 9 A

Fig. 6 Instantaneous number of bonds of each bond type, normalized

with the initial number of bonds of that type, versus time, for

a pc = 48 GPa and b pc = 64 GPa


123

such that it falls within the first-nearest neighbor atom

cutoff. This results in the steep increase in Ni–Ni bonds in

region I (Fig. 6). The amorphous structure of the Ni–Si and

Si–Si bond types results in a gradual deformation that

increases with increasing contact pressure, illustrated in

Fig. 6a and b. Limited deformation is observed in the Si–C

interface and negligible deformation in the ta-C layer.

However, the normalized number of Si–C bond is less than

one after the combined loading procedure, indicating that

this interface has been weakened by the loading. We do not

observe wear or delamination of the DLC coating, which is

likely due to the extremely short duration of the

simulations.

Figure 7 shows the instantaneous number of bonds for

each bond type in the MD model, normalized with the

initial number of bonds of that type, as a function of time

during two combined loading and unloading cycles for a

DLC coating with tSi = 3 A and tC = 12 A, and for con-

tact pressure pc = 48 GPa. Although the number of bonds

during loading is similar in the first and second loading

cycles for most bond types, we observe that the number of

Si–C bonds decreases with repeated loading/unloading

cycles, weakening the interface. The normalized number of

Si–C bonds remains \1 throughout the entire second

combined loading/unloading procedure and results in a

further loss of nearly 1 % of the Si–C bonds. Hence, this

indicates that the Si–C damage is irreversible and will

eventually lead to wear and delamination of the ta-C layer.

Figure 8 shows the mean strain in the recording head

during combined loading, i.e., calculated during region II

in Fig. 6, as a function of tSi for different values of tC and

for pc = 48 GPa (Fig. 8a) and 64 GPa (Fig. 8b), respec-

tively. The mean strain is negative due to compressive

normal loading, and the magnitude increases with

increasing contact pressure. The mean strain during loading

becomes increasingly negative with increasing tSi, similar

to the results for the mean residual strain, which decreases

with increasing tSi. The higher residual tensile strain in the

coatings with thinner tSi counteracts the compressive

loading, decreasing the deformation caused by compressive

loading. Hence, coatings with higher residual tensile strain

show less compressive strain during compressive loading.

The magnitude of the mean strain during loading also

decreases with increasing tC, indicating that a thick carbon

layer prevents deformation of the substrate. Due to the high

cohesive energy of C–C bonds (see Table 1), it can absorb

a large amount of energy compared to the rest of the

coating without significantly deforming.

3.3 Permanent Deformation

Permanent deformation of each bond type in the recording

head is quantified as the final number of bonds after

combined loading relative to the initial number of bonds of

that type. Figure 9a–c show the normalized final number of

Ni–Si, Si–Si, and Si–C bonds, respectively, as a function of

tSi, for different values of tC and for pc = 64 GPa. In each

case, the final number of Ni–Si and Si–Si bonds increases

by 0–6 % compared to the initial number of bonds, signi-

fying strengthening of the Ni–Si interface. The final

number of Si–C bonds changes by ±2 % compared to the

initial number of bonds, indicating that in certain cases, the

Si–C interface is strengthened, and in other cases, it is

Fig. 7 Instantaneous number of bonds of each bond type, normalized

with the initial number of bonds of that type, versus time, for

pc = 48 GPa during two cycles of combined loading and unloading

Fig. 8 Mean strain in the recording head during combined loading as

a function of tSi for different values of tC and for a pc = 48 GPa and

b pc = 64 GPa


123

weakened by the combined external loading. During sim-

ulations with two combined loading cycles, such as the

results shown in Fig. 7, we observe a decrease in the

number of Si–C bonds after two loading cycles compared

to one, indicating permanent deformation that accumulates

each loading cycle and eventually will lead to wear and

delamination between the Si and ta-C coating layers. The

permanent deformation of the Ni–Si, Si–Si, and Si–C

bonds seems nearly independent of tC and tSi for the cases

investigated, especially in light of the stochastic nature of

the MD simulations. The remaining bond types in the

model, Ni–Ni and C–C, show negligible permanent

deformation and are therefore not shown in Fig. 9. The C–

C bonds have the highest cohesive energy compared to the

other bond types, and, thus, no permanent deformation is

observed. The increase in the Ni–Ni bonds during loading,

due to the temporary close proximity of second-nearest

neighbor atoms, is purely elastic and fully recovered once

the compressive loading is removed and the Ni atoms relax

back into their equilibrium positions in the FCC lattice. It is

the amorphous structure of the Ni–Si and Si–C interfaces

as well as the Si layer in combination with the relatively

low cohesive energies of those bond types that cause the

only significant permanent deformation to occur in the Ni–

Si and Si–Si bonds and to a lesser extent, Si–C bonds.

Figure 10 shows the normalized final bond energy as a

function of tSi for different values of tC and for pc = 64

GPa, normalized with the initial bond energy. It is a

measure of the total permanent deformation in the coating,

i.e., the change in energy due to the deformation of the

individual bond types shown in Fig. 9a–c. The final bond

energy increases with increasing tSi and decreasing tC. The

increase in final bond energy with increasing tSi indicates

that a thicker Si layer deforms more than a thinner Si layer

at a given contact pressure. This deformation is due to the

Si layer being the amorphous layer with the lowest cohe-

sive energy. The increase in final bond energy with

decreasing tC agrees with the results shown in Fig. 8 and

indicates that increasing tC improves protection of the

substrate due to the high C–C cohesive energy. The effect

of tC increases with increasing tSi, which indicates that

decreasing the ta-C layer has a bigger effect with increas-

ing Si layer thickness. Thus, to optimize the strength of a

DLC coating for a given coating thickness budget, it seems

more effective to decrease tSi before decreasing tC.

4 Conclusions

We have investigated the deformation of the DLC coating of a

magnetic recording head during combined normal and tan-

gential loading against a moving, rigid, hydrogen-terminated

diamond counterface, simulating accidental contact. The

Fig. 9 Final number of a Ni–Si bonds, b Si–Si bonds, and c Si–C

bonds as a function of tSi for different values of tC, for the case of

pc = 64 GPa. The final number of bonds has been normalized with

the initial number of bonds

Fig. 10 Final bond energy normalized with the initial bond energy as

a function of tSi for different values of tC


123

mean residual strain in the recording head decreases with

increasing tSi, but is independent of tC, indicating that that a

thicker Si layer is desirable for reducing residual strain in the

recording head. Deformation during combined loading and

sliding occurs primarily in the Ni–Ni, Ni–Si, and Si–Si inter-

actions and is a function of the cohesive energy and atomic

structure of the layers. This deformation increases with

decreasing tC, indicating that a thicker carbon layer is desirable

for protecting the recording head during combined loading and

sliding contact. Permanent deformation is observed primarily

in the Ni–Si and Si–C interfaces and in the Si layer and is also a

function of the cohesive energy and atomic structure of the

layers. Permanent separation between material layers is only

observed in the Si–C interface and increases with additional

combined loading cycles. The total permanent deformation of

the DLC coating increases with increasing tSi and decreasing

tC. Hence, to minimize deformation of the DLC coating under

combined loading, for a given coating thickness budget, it is

preferable to decrease the thickness of the Si layer before

decreasing the thickness of the ta-C layer.

Acknowledgments The authors thank Western Digital Corporation

for their support of this research, and Dr. Min Yang for her interest in

this work. The support and resources from the Center for High

Performance Computing at the University of Utah are gratefully

acknowledged. We also acknowledge use of AtomEye [44].

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Deformation of Ultra-Thin Diamond-Like Carbon Coatings